One of the problems faced by an automobile driver at night is the poor visibility of road signs, and also of road stripes, variously called road delineators, delineating stripes, pavement markings and roadway markings. It has long been a goal of governmental and municipal authorities responsible for road safety to make figures and letters on road signs, and road stripes brighter or clearer in the headlights of oncoming traffic. Initially, this has called for using bright paints or reflective surfaces capable of reflecting the light of the headlights of oncoming cars. Paints have been developed which are highly reflective. Such paints usually comprise a titanium dioxide constituent. Unfortunately, the major drawback of such reflective surfaces is, unless the reflective surface is substantially normal to the beams from the headlights of the oncoming car, the vast majority of the impinging light from the headlight beam is deflected or reflected in a direction or directions other than back at the oncoming car.
A further problem occurs when poor night visibility is aggravated by rain, snow or fog. Visible light from headlights backscatters under these conditions, creating glare, so that sign or marker visibility cannot be improved by using brighter headlights.
Other paints have been developed which additionally fluoresce at optical wavelengths when irradiated with optical wavelength light, such as that produced by conventional headlights, and these paints have been used in road signs and road markers. While such signs and markers provide more light for the night driver to see by than a conventional sign or marker, these paints suffer from the drawback that in order to provide a substantial amount of light by fluorescence, it is desirable to increase the light output of impinging auto headlights, which then creates an additional glare hazard for traffic traveling in the opposite direction. As a consequence, such fluorescent paints are preferably used only in situations where oppositely-directed traffic is not in the headlight beam path.
In an effort to circumvent some of these difficulties, sign and marker designers and makers began in the early 1980's to incorporate retroreflective devices in signs and road markers. A retroreflector has the property of reflecting a substantial portion of impinging light back generally in the direction it came from. A typical retroreflector comprises a sphere or bead, typically made of glass or clear plastic. Retroreflection occurs by the tandem action of refraction of the light through the upper surface of the sphere or bead, reflection from the lower inside surface of the sphere or bead, and subsequent refraction of the light as it exits from the upper surface of the sphere or bead, back in the direction from which the impinging light came. A substantial portion of the light is retroreflected, with possibly only nominal losses for some reflection at the upper surface and refraction at the lower surface.
A retroreflector may also comprise a cube corner reflector. In such a retroreflector, the arrangement of three orthogonal, reflective surfaces to form a cubical corner guarantees that light impinging on the concavity is reflected back substantially along a path parallel to the path of the impinging light.
Minute glass particulates have been incorporated into paints to make road lines and road lane delineators brighter at night under headlights. Such particulates, especially in the form of small beads, act in a retroreflective fashion. One such product comprising glass beads having diameters of about 1.5 millimeters is called Visibead, sold by Potters Industries, N.J.
Signs have been constructed by stamping large, flat aluminum sheets with a pattern of tiny tetrahedral indentations which form prismatic or cube corner retroreflectors. A thin sheet of colored plastic is adhesively attached over the aluminum sheet to provide a colored sign surface capable of at least some degree of retroreflection.
A drawback of these retroreflective devices is that they still do not adequately overcome the problems created by rain, snow or fog. Backscattering reduces the overall amount of light from headlights before the light even gets to the retroreflector, which is capable of reflecting at best only as much light as impinges thereon. Furthermore, a water layer may form on the surface of retroreflectors during these adverse weather conditions, reflecting a substantial portion of the impinging light in a scattering fashion, thereby diminishing the amount of light which reaches the retroreflector for retroreflection.
Most recently, steps have been taken in Sweden to install road markers which comprise posts having fluorescent properties responsive to ultraviolet wavelengths. These posts are placed at intervals along the side of the road to indicate the curvature of the road. This requires the utilization in Sweden of headlights in cars which emit ultraviolet light. This has the advantage of providing substantial activating energy for fluorescence, while not blinding oppositely-directed traffic, since humans cannot see ultraviolet wavelengths. However, since the posts are emplaced at intervals, they necessarily leave gaps between, and furthermore are subject to obstruction by plants or debris at the roadside. Furthermore, the posts do not provide retroreflection.
There is a need in the industry for a road sign or road marker which provides sufficient light to be safely visible and clear to oncoming traffic during the night, regardless of weather conditions. More particularly, there is a need to utilize as much as possible of impinging headlight energy to provide an adequately bright and clear display or marker, without requiring impractically bright headlights. Such road signs and road markers are preferably able to remain bright and clear even under adverse weather conditions, and furthermore, should be constructed to remain as bright and clear as possible for extended periods of time, so that they do not require frequent replacement or revitalization.